Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant disorder characterized by ataxia, epilepsy and anticipation. A large expansion of the ATTCT repeat in intron 9 of the SCA10 gene encoding the E46L protein is the mutation responsible for this disease. The mechanism by which the expansion mutation leads to the disease phenotype is unknown. The goal of this project is to elucidate the pathogenic mechanism of SCA10. The long-term goal is to develop a rational therapy for SCA10 based on the pathogenic mechanism. The central hypothesis is that the major pathogenic mechanism of SCA10 is due to the disruption of the normal cellular function of polyprimidine tract binding proteins (PTBs) by the large expanded AUUCU repeat in the E46L RNA. This hypothesis is based on preliminary data that (i) SCA10 cells do not show altered levels of E46L mRNA and protein, (ii) loss of function of the E46L protein does not lead to a SCA10-like phenotype in mice, (iii), expanded ATTCT repeats are transcribed and accumulated in nuclear foci, and (iv) three species of PTBs (PTB, nPTB and hnRNP K) bind to AUUCU repeats in vitro and at least one (hnRNP K) co-localizes in the nuclear foci in cells derived from an SCA10 patient. The following two specific aims will be used to test our hypothesis. Specific Aim 1 is to demonstrate that the transcript with an expanded AUUCU repeat gains toxic functions by binding and perturbing the function of PTBs in SCA10 cells. It is expected that (a) there is a detectable level of the expanded AUUCU repeat in SCA10 cells, and (b) expanded AUUCU repeats in SCA10 cells bind to PTBs and alter the cellular functions of PTBs. Specific Aim 2 is to establish genetic mouse models of SCA10. Aim 2 will be accomplished by (a) identifying the SCA10-like phenotype in the transgenic mice that express expanded AUUCU repeats, and (b) establishing genetic mouse models of SCA10 that lack the Hnrpk gene encoding hnRNP K and the Ptbp2 gene encoding the brain-specific nPTB. Transgenic mice expressing expanded AUUCU repeats are expected to exhibit an SCA10-like phenotype by the RNA-mediated gain of function through deficiency of these PTBs. Elucidating the disease mechanism of SCA10 is innovative because SCA10 is the only human disease known to be caused by an expansion of a pentanucleotide repeat. Establishing the mechanism in SCA10 will fill the current gap of knowledge and facilitate the development of rational therapy.